To provide frost-safe cooling of engineering equipment, in particular of motor vehicle engines, use is made, as is known, of liquids based on glycols, especially ethylene glycol andlor propylene glycol. In order to protect the materials of the cooling system against corrosion, these liquids contain effective amounts of one or more corrosion inhibitors. For use in cooling systems, they are diluted with water and are intended to provide not only protection against freezing and corrosion but also good heat removal. Modern internal combustion engines subject the cooling medium to more severe operating conditions because of the higher temperatures of the metal surfaces, the increased flow rate of the cooling medium and the choice of materials. The increased use of light alloys based on aluminum together with steel, cast iron, brass, copper and soft solder in a mixed construction mode also requires careful matching of the inhibitor combination used. In recent decades, innumerable combinations of this type have been described in the relevant patent literature, so that numerous inhibitor systems having specific mechanisms of action are known. Thus, the previously customary nitrite- and silicate-containing combinations comprising nitrites, nitrates, borates, phosphates, benzoates, triazoles and silicates (e.g. H. D. Held, Kuhlwasser, 2.sup.nd edition, pp. 213-225 (1977)) have been replaced by nitrite-free preparations generally based on monocarboxylic and dicarboxylic acids (e.g. EP-A-0 035 834), but these still contained silicates as effective aluminum inhibitors, even if in stabilized form.
It has long been known that borates can greatly impair the corrosion performance of aluminum under heat transfer conditions. Silicates can prevent this, so that a balanced ratio can be achieved under optimal conditions. In practice, it has been found that certain operating conditions can lead to a reduction in the silicate content, e.g. due to precipitation of insoluble silicate modifications at very high temperatures, even when the silicates are stabilized, for example by means of organosilicon compounds. This led to the development of silicate-free formulations which have a significantly longer life since they do not have this disadvantage; see EP-A-0 816 467.